1. Technical Field
The present invention pertains to a method and apparatus for optimizing plant process operations such as, but not limited to, alumina refining, fractionation, cryogenic expansion and gas processing and treating. More particularly, the invention is directed to a method and apparatus for improving feedforward, plural variable control techniques in plant process operations.
2. Discussion of the Prior Art
As pointed out in U.S. Pat. No. 4,349,869 (Prett et al), it is important to minimize losses inherent in the processes performed in industrial plants, and it is likewise important to simultaneously manage profits. Prett et al recognize that feed-forward control is important to the optimization process because it permits the user to initiate controller action based upon a prediction of the values of controlled variables. The patent points out that prior feed-forward controllers have certain inherent problems based on the fact that the controllers have no "knowledge" as to what effect their controlled condition will have elsewhere in the overall process, thereby requiring operator intervention to alleviate problems. In addition, prior systems are described as being unable to handle "large scale" feed flow disturbances, on the order of 10 to 15%, that are considered usual in petroleum cracking processes, for example. As a solution to that problem, Prett et al provide a method for controlling and optimizing operation of a process having plural input variables, plural independently manipulated variables and plural controlled variables that are dependent on the input variables and the manipulated variables. The input variables themselves--e.g., input flows, compositions, etc. may or may not be subject to manipulation but are classified as manipulated variables for the purposes of this discussion. The method involves introducing test disturbances in the manipulated variables and then measuring the effect on the controlled variables, thereby permitting the response characteristics of the controlled variables (to a given change in one of the manipulated variables) to be readily calculated. The existing values of the manipulated variables and the controlled variables can then be measured, and the calculated response of the controlled variables may be used to calculate a new set of moves for the manipulated variables. The manipulated variables can then be adjusted in accordance with the new set of moves to reach a new set of values. These moves, when implemented, have the effect of moving the controlled variables toward their optimum setpoints. A feature of the Prett et al system is that it allows the formation of a projection to some future time of future controlled variable values. In the same manner, a number of future moves of each manipulated variable may be calculated to control the future values of the controlled variables to their desired operating points. Thus, feed-forward control is implemented by predicting, at one or more points in the future, the response of a process to changes in the manipulated variables. Based on the predicted trend of the process, a number of future moves for the manipulated variables can be calculated to minimize the error between the desired setpoint and the predicted future response of the process. Importantly, feedback is used to predict--and hence minimize--setpoint error.
The system described above has a number of limitations, not the least of which is an inability to respond to truly large scale variations in the feed-forward parameters such as throughput and composition, and to volatile economic conditions affecting the system. In this context, "truly large scale" means variations on the order of one hundred percent or more. For example, the system disclosed in the Prett et al patent performs a cracking process in an oil refinery. The raw material (e.g., crude oil) is typically available in storage tanks containing two to three weeks supply for the system, thereby enabling the system operator to control the inflow rate and composition within fairly narrow limits. However, in gas processing fractionation systems, or in other systems wherein large supplies of the raw material are not capable of storage, such operator control is hardly feasible. Considering the gas fractionation situation, the incoming gases typically arrive directly by pipeline from remote locations, usually without any control by the plant operator as to the time, rate of arrival, or composition of the total stream. The result is a large and random variation at the front end of the system, i.e., in the raw material. Similarly, at the back end of the plant there are typically no storage tanks for the output product as in the case of petroleum cracking processes; rather, the outflow from the gas fractionation process (typically cleaned gas and natural gas liquids) proceeds directly to the pipeline.
Processes such as gas fractionation are also affected by economic parameters that are much more volatile than economic parameters affecting the final product of the Prett et al system. Federal regulation has relegated the gas transportation industry to a common carrier status, effectively precluding the carriers from contracting to purchase gas from the producers and then contracting to sell it downstream to users. As a consequence, most gas is sold on a spot market characterized by significant and frequent price variation. Although the system described in the Prett et al patent can accommodate changes in economic conditions, it can only do so by being shut down and being reconfigured to function with changed parameters. Such down time and off-line reconfiguration is totally impractical for industries such as the natural gas industry where economic parameters are extremely volatile.
In a fundamental sense, the Prett et al model is limited in that it is only mechanistic rather than based on the physics and chemistry of the processes being controlled.